UK Shale Gas: Using Geochemistry and Biostratigraphy to Correlate Wells in the Upper Bowland Shale Formation

Abstract

The Pendleian Upper Bowland Shale Formation (UBSF) is a target for UK shale gas extraction. Reliable well correlation is limited due to grain size homogeneity, but needed as folds and extensional faults increase uncertainty. Currently, maximum flooding surfaces - so called ‘marine bands’ - are used as stratigraphic marker beds. Marine bands are enrichments of specific goniatite index fossils, formed during periods of enhanced fossil preservation. As UBSF goniatite fossils are poorly preserved and challenging to identify, organic and inorganic geochemical analyses are used alongside biostratigraphy. However, the exact relationship between marine band geochemistry and basin palaeoceanography is poorly understood.

Here we present a lateral comparison of sedimentological, palynological, biomarker and elemental data across two cores (from north west UK) and at outcrop (Clitheroe), transecting the palaeobasin. Marine bands across this transect are rarely bioturbated. RockEval, optical and palynological analyses suggest that the total organic carbon (TOC) is mostly comprised of amorphous organic matter (AOM) consisting primarily of Type II/III kerogen. Marine bands are not always connected to high TOC values, however solvent extractable organic matter (EOM) is typically most abundant within a marine band. Gas chromatography mass spectrometry (GC/MS) molecular marker analyses indicate that the marine bands were deposited during the periodic occurrence of photic zone anoxia. This indication is supported by the associated abundance of redox sensitive trace metals (e.g. U, V and Mo).

We show that processes controlling marine band formation are 1. not fully represented by the goniatite fossils 2. occur over a longer period of time than is represented by the goniatite fossils 3. are associated with the periodic establishment of photic zone anoxia and water column stratification. It is only by using high-resolution geochemistry that we can improve the reliability of biostratigraphy and increase the accuracy of UBSF well correlation.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018